Due to the high thermal efficiency achieved by compression-ignited engines (e.g., in comparison with spark-ignited engines), these engines are commonly utilized in industrial applications. The high efficiency of compression-ignited engines, such as diesel engines, is due in part to the ability to use higher compression ratios than spark-ignited engines (i.e., gasoline engines) as well as the ability to control power output without a throttle. In the latter regard, the lack of a throttle eliminates throttling losses of premixed charges typical in spark-ignited engines thereby resulting in significantly higher efficiency at part load. However, compression-ignited engines and diesel engines in particular typically cannot achieve the low oxides of nitrogen (NOx) and particulate emission levels that are possible with spark-ignited engines.
Diesel engines typically inject diesel fuel into the engine's combustion chamber when that chamber's piston is near the end of the compression stroke. The high pressure present in the chamber ignites the diesel fuel. Due to this mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel-rich equivalence ratio. That is, the fuel and air in the combustion chamber are not necessarily a homogenous mixture. This may result in incomplete combustion of the diesel fuel, which tends to result in high particulate emissions. Furthermore, the fuel-rich equivalence ratio can also lead to high flame temperatures in the combustion process, which results in increased NOx emissions. As tougher environmental standards are being enacted for diesel sources, users of diesel engines are looking for ways to lower emissions. One solution is to reduce the amount of diesel injected into the combustion chamber, which reduces the equivalence ratio and works to reduce particulate and NOx emissions, however, it also reduces engine power.
In order to reduce particulate and NOx emissions levels from diesel engines, such engines may also be partially or completely converted for use with gaseous-fuels such as, compressed natural gas (CNG), liquid natural fuels (LNG) such as ethanol, and liquid or liquefied petroleum gas (LPG), such as propane. Utilization of such gaseous-fuels with diesel engines not only provides for more complete combustion and thereby enhanced fuel economy, but also typically results in lower engine emissions. However, gaseous-fuels typically do not have the centane value required to allow for their ignition through compression. Accordingly, diesel engines must be modified to use such fuels. Methods for converting a diesel engine to consume gaseous-fuels typically fall into three categories. The first is to convert the engine to a spark-ignited engine; a second is to convert the engine to allow for the direct injection of gaseous-fuels into the combustion chamber with injected diesel; and a third is a dual-fuel technology, in which the gaseous-fuel is mixed with all or a portion of the intake air of the engine. As will be appreciated, the second and third methods utilize injected diesel (i.e., pilot diesel) to ignite the gaseous-fuel. In this regard, the combustion of the gaseous-fuel results in more complete combustion of the diesel. Furthermore, as the gaseous-fuel allows the engine to produce additional power, less diesel is injected into the cylinders.
Conversion to a spark-ignition system and/or a direct gaseous-fuel injection system for utilizing gaseous-fuels with a diesel engine each typically require substantial modification to the diesel engine. Such modifications may include replacement of cylinder heads, pistons, fuel injection system and/or duplication of many engine components (e.g., injection systems). Accordingly, these systems are typically expensive and oftentimes unreliable. On the other hand, dual-fuel systems require little modification to existing engines.
Duel-fuel operation where gaseous-fuels are mixed with intake air prior to the introduction of that air into the engine is known in the art as fumigation. The mixture of gaseous-fuel and intake air is introduced into each cylinder of the engine during the intake stroke. During the compression stroke of the piston, the pressure and temperature of the mixture are increased. Near the end of the compression stroke, a small quantity of pilot diesel fuel from the engine's existing diesel fuel injection system is injected into the cylinder. The pilot diesel ignites due to compression and in turn ignites the mixture of gaseous-fuel and intake air. As will be appreciated, such fumigation systems may be retrofit onto existing diesel engines with little or no modification of the existing engine. Furthermore, engines using such fumigation systems may typically be operated in a dual-fuel mode or in a strictly diesel mode (e.g., when gaseous-fuel is not available).